1. Field of the Invention
The present invention relates to an exhaust gas purifying apparatus containing an exhaust pipe for forming an exhaust way communicated with an exhaust port of an engine, and a catalyst disposed in the exhaust way for purifying exhaust gas.
2. Description of the Related Art
Conventionally, in a small size engine such as a motorcycle engine, a cylindrical catalyst is used for purifying exhaust gas discharged from an exhaust port of a motorcycle engine to an exhaust way of an exhaust pipe. This cylindrical catalyst contains a punching tube with a center hole and a catalyst component retained on the punching tube. This cylindrical catalyst is disposed to cover an inwall surface of the exhaust way for gaining an engine output sufficiently. This cylindrical catalyst is high in a blowing rate so as to suppress a pressure loss for ensuring the engine output. This cylindrical catalyst, however, is a small area to react with the exhaust gas, exhibiting a low purification-rate. So, this cylindrical catalyst sometimes requires the honeycomb catalyst at the downstream of the cylindrical catalyst in the exhaust way for improving the purifying ability.
Further, Patent Publications disclose some exhaust gas purifying apparatus with the cylindrical catalyst which retains a catalyst component on a punching tube in following (1)–(5).
(1) Japanese Unexamined Patent Publication (KOKAI) No.10-299,469 (1998) discloses an exhaust gas purifying apparatus with a catalyst pipe in which a catalyst component is retained on an inner circumferential surface and on an outer circumferential surface of a punching pipe capable of swinging in the central region located in a radial direction of the exhaust way of the muffler of the 2-stroke cycle engine. For activating the catalyst component, it is desirable that the catalyst pipe is heated over the predetermined temperature. Accordingly, the catalyst pipe is moved to an upstream side in the muffler for activating the catalytic reaction, when temperature in the muffler is low like an engine-idling period. The catalyst pipe is moved to the downstream side in the muffler to suppress overheat of the catalyst, when the temperature in the muffler is high. In this apparatus, the number of catalyst pipe is only one.
(2) Japanese Unexamined Patent Publication (KOKAI) No. 5-312,030 (1993) discloses an exhaust gas purifying apparatus in which a cylindrical catalyst retaining a catalyst component is installed at a central portion of an exhaust way of the muffler by a holding plate having a disk-shape. This apparatus has a blowing hole in the holding plate to discharge exhaust gas. So, the exhaust gas runs into a cylindrical catalyst. The remainder of the exhaust gas runs not through the cylindrical catalyst but through the blowing hole of the holding plate. In this apparatus, the number of catalyst pipe is only one.
(3) Japanese Unexamined Patent Publication (KOKAI) No. 7-54,642 (1995) discloses an exhaust gas purifying apparatus which contains: an exhaust pipe connected with the exhaust port of the engine, and an exhaust muffler connected with the downstream side of the exhaust pipe. This apparatus has the cylindrical main catalyst member disposed coaxially to cover the inwall surface of the exhaust pipe. The exhaust gas is purified by the main catalyst member when the engine is driven. However, when the main catalyst member is not sufficiently heated in an engine starting period, the activation of the main catalyst member is insufficient. Then, this apparatus is provided with a sub catalyst member, namely, a cylindrical portion retaining the catalyst component, at the portion connected with the exhaust port of the engine in the exhaust pipe. The sub catalyst member is early heated to be activated in the engine starting period, since it is near to the exhaust port of the engine. Accordingly, the purification rate is improved in the engine starting period.
(4) Japanese Unexamined Patent Publication (KOKAI) No. 7-269,331 (1995) discloses an exhaust gas purifying apparatus including a punching pipe with a catalyst component and covering an inner surface of the exhaust pipe coaxially.
(5) Japanese Unexamined Patent Publication (KOKAI) No. 5-86,843 (1993) discloses an exhaust gas purifying apparatus in which the main catalyst 100 for purifying the exhaust gas is placed at a downstream side of an exhaust way 220 of a body 210 connected with the exhaust pipe 200. The apparatus shown in FIG. 10 has a pre-catalyst 300 placed in an upstream side of the main catalyst 100. A blowing passage 400 is formed between an outer circumferential surface of the pre-catalyst 300 and an inwall surface of the exhaust way 220. The pre-catalyst 300 is formed of a ceramic honeycomb carrier 301 retaining a catalyst component, and it is held in an external sleeve 302 by cushions 303 for preventing the damage of the ceramic honeycomb carrier 301 as shown in FIG. 11. The pre-catalyst 300 is placed by a supporting member 330 (a width of “M”, a thickness of “t”) in the center region located in a radial direction of the exhaust way 220. The apparatus shown in FIG. 10 is provided with a closing plate 212 for reflecting the exhaust gas. The closing plate 212 is placed distance “LA” apart at the downstream side of the main catalyst 100 in the body 210 so as to face the flow of the exhaust gas.